'Salmon Mousse' is a fish dish mixed with whipped cream or gelatin.
This Mousse is highly sort after by a french organisation which is translated to 'The Anti Fish League', this was set up after a number of cold fish dishes turned on thier consumers and tried to kill them.

'Salmon Mousse' can also be used to express how stupid you think someone is, thus comparing someone to the phrase 'thick as two shits'.